It is known to react tetrachlorocyclobutenone with butanol at 120.degree. C. to produce butyl 2,3,4,4-tetrachloro-3-butenoate (Liebigs Ann. Chem. 686, 55 (1965)). The tetrachlorocyclobutenone required as the starting material can be manufactured in a simple manner by a process which is not part of the prior art; specifically, by reacting (a) hexachlorocyclobutene with (b) anhydrous sulfuric acid and (c) sulfur trioxide; or with (b) anhydrous sulfuric acid and (d) phosphorus pentoxide at a temperature of 30.degree.-140.degree. C. The starting materials are present in the following molar ratios at the start of the reaction:
(a) : (c)=0.67 to 10, PA1 (b) : (c)=0.1 to 10, PA1 (a) : (d)=0.67 to 10 and PA1 (b) : (d)=0.5 to 20.
This reaction is the subject of copending U.S. Application Ser. No. 019,204, filed on Mar. 9, 1979 whose disclosure is incorporated by reference herein.
Hexachlorocyclobutene, in turn, can be obtained by isomerization of hexachloro-1,3-butadiene (German Offenlegungsschrift No. 2,618,557). Because of the small difference in the boiling points of the two substances (4.7.degree. C. at 67 mbars), this isomerization, which in general is carried out by distillation, requires an extremely high expenditure on apparatus. It is therefore considerably less expensive to instead work with mixtures of hexachlorocyclobutene and hexachloro-1,3-butadiene.
Since the reaction of hexachlorocyclobutene with, for example, fuming sulfuric acid (oleum) by the above-mentioned process proceeds equally well in the presence of hexachloro-1,3-butadiene as does the reaction of highly pure hexachlorocyclobutene, the less expensive mixture can be advantageously employed in preparing the tetrachlorocyclobutenone. However, of course, the result is a mixture of tetrachlorocyclobutenone and the hexachloro-1,3-butadiene.